1. Field of the Invention
The present invention relates to techniques for preventing reduction of the service life of a discharge lamp in a vehicle lighting device using the same lamp.
2. Description of the Related Art
A discharge lamp, such as a metal halide lamp, is used for a small light source for a vehicle. A lighting circuit is used for lighting on such a light source. A known lighting circuit includes a DC/DC converter circuit, a DC/AC converter circuit and a starter circuit.
Where discharge lamps are used for a light source for the running beam (called the high beam) and a light source for the dipped beam (called the low beam), two types of lighting circuits are known for lighting the discharge lamps. A first lighting circuit contains lighting circuits respectively for lighting the discharge lamps. A second lighting circuit contains common circuits used in common for the two discharge lamps. The common circuits are, for example, the DC/DC converter circuit and the DC/AC converter circuit. The latter lighting circuit is advantageous in cost and installing space.
In either lighting circuit, where it is designed such that the driver may give an instruction to substantially simultaneously light on both the high-beam and low-beam discharge lamps, the driver tuns on a lamp lighting switch in a state that the operation lever is set to the low beam side, or turns on or off for a short time by operating the lever, switch or the like.
In the conventional vehicle lighting device, when the high-beam discharge lamp and the low-beam discharge lamp are substantially simultaneously lit on, inconvenience is present which is caused by indefinite rules on which of the high-beam discharge lamp and the low-beam discharge lamp is first turned on.
For example, in a vehicle lighting device using the high-beam discharge lamp and the low-beam discharge lamp, which designed to allow an operation to light on both the discharge lamps, and an operation to light on only the low-beam discharge lamp, difference will occur between those discharge lamps in the use time and the number of times of lighting the discharge lamps. Where the discharge lamps are used for producing the high-beam and the low-beam, it is a rare case that the high-beam discharge lamp and the low-beam discharge lamp are used at the equal number of times. Generally, the driver more frequently uses the low-beam discharge lamp. The same thing is true for the whole lamp lighting time and the number of lamp lighting operations. Therefore, the service life of the low-beam discharge lamp, rather than the high-beam discharge lamp, must be taken into consideration. In other words, the service life of the high-beam discharge lamp is sufficiently long in the light of the use frequency of the discharge lamp.
The lighting time and the number of lighting operations may be enumerated for the factors to determine the reduction of the service life of the discharge lamp if the discharge lamps are under the same power inputting conditions. Electric power higher than the rated power is input to the discharge lamp in order to improve a light flux rising characteristic when a discharge lamp lighting device is started. Accordingly, as the number of lighting times is larger, the necessity of replacing the discharge lamp with a new one, which result from life deterioration, occurs before the usual replacing time.
When the serving time and the use frequency of the low-beam discharge lamp becomes considerably large when comparing with the high-beam discharge lamp, the life deterioration of the discharge lamp is remarkable. In particular, where the flashing operation, such as passing, is repeated many times, the service life of the discharge lamp is reduced.
To cope with this, the following measure may be taken. When the high-beam discharge lamp is lit on in a state that the low-beam discharge lamp is lighting, there is no need of turning on the two discharge lamps. According to the rule, only the high-beam discharge lamp is light on, while the low-beam discharge lamp is lit off. In this case, when the high-beam discharge lamp is lit off, the low-beam discharge lamp must be lit on. As a result, the number of lighting times increases, and the life deterioration progresses.
Accordingly, an object of the present invention is to take a measure for increasing the service life of the discharge lamps in a vehicle lighting device having a lighting circuit in which discharge lamps are used for the high- and low-beam irradiation.
According to an aspect of the invention, there is provided a vehicle lighting device which uses a high-beam discharge lamp and a low-beam discharge lamp, and includes a DC/DC converter circuit for converting an input voltage output from a DC power source into a desired DC voltage, a DC/AC converter circuit for converting an output voltage of the DC/DC converter circuit into an AC voltage, starter circuits for applying start pulse signals to the discharge lamps, and a control circuit for controlling the lighting of the discharge lamps by detecting voltage or current of each arch discharge, wherein when an instruction to light on the high-beam discharge lamp and the low-beam discharge lamp is given, and the high-beam discharge lamp is lit on in a state that the low-beam discharge lamp is lit off, the low-beam discharge lamp is lit on with some time delay.
According to another aspect, the vehicle lighting device thus constructed is characteristically featured in that when one of the high-beam discharge lamp and the low-beam discharge lamp is lighting, the other discharge lamp is lit off.
The invention reduces the lighting time and the number of lighting times thereby preventing the reduction of the service life of the discharge lamps, and the replacement frequency.